Heretofore, machine readable markings have been provided utilizing a related sequence of substantially parallel bars or stripes of predetermined widths and spacings which are commonly referred to as bar code markings. The marking may utilize bars of one-unit and three-unit widths, and spaced therebetween of one-unit and three-unit widths. Bar, codes have been described in detail elsewhere.
Glass is a ceramic material consisting of a generally uniformly dispersed mixture of silica or sand (75%), soda ash (20%), and lime (5%) often combined with such metallic oxides as those of calcium, lead, lithium, cerium, etc. depending on the specific properties desired. The blend is heated to fusion temperature around 700.degree.-800.degree. C. (1292.degree. F.-1472.degree. F.) and then cooled or annealed to a rigid or vitreous state. While bar code markings may be placed anywhere on glass products or glassware, it is important that markings be placed at locations that are easily located and accessible. It is desirable that such bar code marking be permanently secured or integral with the glassware so that the bar code is inseparable from the glassware.
One method utilized heretofore for providing a machine readable bar code marking on glass is shown in U.S. Pat. No. 4,600,630 dated Jul. 15, 1986. The bar code is applied to the glass by first depositing a thin undercoating of light-modulating material on the glass surface, then recessing or etching selected portions through the coating to define the machine readable marking, next integrating the marking into the surface of the glass by baking, and then applying a protective overcoating. Thus, the bar code marking is first etched into the glass surface and then a ceramic glaze overcoats the marking as a protective coating. The undercoating and overcoating when heated or fired are integrated into the main body and become inseparable. Thus, the coating cannot be removed without destroying the coatings. Such a process is relatively complicated utilizing two separate coatings and normally requiring separate heating of the coatings.